Exhaust rattles

["Quinn, Thomas P." <QuinnT@domain.elided>]

My 88 Alfa spider has an unmistakable rattle in the area of the catalytic
converter around 1500 RPM and 2500 RPM. It all started the other day when I
was passing a cement truck. I was in second at the time and pulled out and
shifted into third around 5000 RPM. As I was making the shift I heard an awful
noise like some threw gravel down the exhaust. When I stopped at the next
light I noticed the car was now idling roughly. The idle speed was fairly
constant at 950 RPM but not smooth like before. Based on what I have read in
previous digests and noting the location of the rattling noise I think I need
a new catalytic converter. Do you think this is a reasonable conclusion? Is
there anything special I should know about purchasing a new catalytic
converted, like a particular brand preference or aversion? Can this job been
accomplished by someone with limited Alfa experience?
I also wanted to know if the catalyst broke free could that be causing the
rough idle by partially blocking the exhaust path? Or, do I need to have the
timing checked and possibly a new oxygen sensor? It roughness smoothes out
above 1200 RPM? Thanks in advance.

Sincerely,

Tom Quinn
Columbia, MO

88 spider veloce
55,400 miles
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